Optical encoders are used to monitor the motion of, for example, a rotary shaft such as a crank shaft. Optical encoders can monitor the motion of a shaft in terms of position and/or number of revolutions of the shaft. Optical encoders typically use a code wheel attached to the shaft to modulate light as the shaft and the code wheel rotate. In a transmissive code wheel, the light is modulated as it passes through transmissive sections of a track on the code wheel. The transmissive sections are separated by non-transmissive sections. In a reflective code wheel, the light is modulated as it is reflected off of reflective sections of the track on the code wheel. The reflective sections are separated by non-reflective sections. As the light is modulated in response to the rotation of the code wheel, a stream of electrical signals is generated by a photosensor array that receives the modulated light. The electrical signals are used, for example, to determine the position and/or number of revolutions of the shaft.
FIG. 1 illustrates a conventional transmissive optical encoder system 10. The optical encoder system 10 includes an encoder 12 and a transmissive code wheel 14. The code wheel 14 is coupled to a rotary shaft 16 of a motor 18. The encoder 12 includes a light source 20, a collimating lens 22, and a detector 24. Together, the light source 20 and the collimating lens 22 also may be referred to as an emitter. The light source 20 emits light, which is collimated by the collimating lens 22 and is modulated as it passes through the transmissive sections of the code wheel 14. The detector 24 includes a photosensor array such as an array a photodiodes which detects the modulated light. Typically, the photosensor array has a resolution that is equal to the resolution of the coding element.
One disadvantage of conventional transmissive optical encoders, compared to conventional reflective optical encoders, is that the placement of the emitter 20 and the detector 24 on opposite sides of the code wheel 14 consumes more space, making the conventional transmissive optical encoder 12 relatively large. Also, misalignment of the emitter 20 and/or the detector 24 with the code wheel 14 or with each other degrades the performance of conventional transmissive optical encoders 12